Hydraulic brakes used to brake the wheels of aircraft generally comprise a ring having multiple cavities in which braking actuators are attached in a removable manner. Each of the actuators comprises a liner which is attached in a sealed manner in one of the cavities of the ring and in which a piston is slidably mounted in a sealed manner along a sliding axis. The ring distributes pressurized hydraulic fluid to all its cavities, which fluid acts on the piston in order to extend it and to apply a braking force on friction elements extending facing the ring, including rotors which turn with the wheel, and stators which are rotationally immobile.
The piston generally has an operational travel which it covers during the application of a braking force. This operational travel, of the order of a few millimeters, is sufficient to allow the application of the braking force on the friction elements and to allow the push rod to retreat so as to permit the free rotation of the rotors. For this purpose, a spring extending within the piston ensures that the push rod is retreated to a retreated position when the braking force is no longer applied.
However, since the friction elements wear progressively subsequent to the repeated application of braking forces, it is important to ensure that the piston is always situated close to the friction elements. To achieve this, it is known practice to equip the braking actuators with a wear-compensating device which extends inside the piston. The wear-compensating device comprises a movable stop which is slidably mounted with friction along the sliding axis on a central rod which extends in the piston and which defines the retreated position of the piston.
During the application of a braking force, the piston is pushed towards the friction elements and, where appropriate, drives the movable stop with it while overcoming the friction between the rod and the stop, thereby causing the movable stop to be advanced on the rod. When the force is released, the spring, which extends between the movable stop and the piston, causes the piston to be retreated to the new retreated position, which has advanced owing to the advancement of the movable stop.
The friction can be organised by various means, such as, for example, the interposition of friction rings between the stop and the central rod, or else the interposition of a deformable sleeve co-operating with an olive which radially deforms the sleeve on each advancement of the movable stop.
There are essentially two technologies for this type of braking actuator, in one of them, termed wet piston, the hydraulic fluid is admitted inside the piston and immerses the wear-compensating system. These actuators are simple, but have the disadvantage that the fluid extends in zones close to the end of the piston which is subjected to high temperatures owing to the heating by friction of the friction elements of the brake. This results in a partial alteration of the physico-chemical characteristics of the hydraulic fluid, which may degrade the braking performance in the long term.
In the other of the technologies, termed dry piston, a seal is interposed between the piston and the central rod, with the result that the fluid remains confined at the rear of the piston and no longer immerses the wear device. However, such a technology requires the use of an additional seal which poses other wear problems in use.